Cracking of alpha-methylstyrenes



Patented Dec. 25, 1945 v UNITED STATES 2,391,821 moms or ALPHA-METHYLSTYRENES James Kenneth Dixon, Riverside, Conn, assignor to American Cyanamid Company, New York, v N. Y., a corporation of Maine No Drawing. Application July 7, 1944, Serial No. 543,967

3 Claims. (Cl. 260-669) Thisinvention relates to the cracking of 4- methyl, alpha-methyl styrene. More particularly, the invention relates to a novel catalytic process of cracking 4-methyl, alpha-methyl styrene will undergo similar reactions when subjected to cracking conditions. Actually, according to the present invention, this is not found to be the case.

whether in straight thermal cracking or in carryto p-methyl styrene in the vapor phase and to a 5 ing out a catalyzed reaction.

. novel catalyst for use in that process.

. In my copending applications'for U. S. Letters Patent, Serial No. 471,875, filed January 9, 1943, and Serial No. 491,939, filed June 23, 1943, the production of p-cymene and alpha, ii-dimethyl styrene from terpenes by catalytic dehydrogenation in the vapor phase has been set forth. Alpha, 4-dimethyl styrene, commonly designated as dimethyl styrene andso referred to in the present specification, is itself a product of great industrial utility for many purposes. For other purposes, p-methyl styrene is required. The latter is particularly useful in the production of copolymers with other polymerizable materials in the manufacture of various plasters, resins and synthetic rubber. p-Methyl styrene, however, is not readily produced directly from the terpenes which serve as the starting materials of my previously mentioned copending applications, which otherwise would constitute a highly desirable source.

It is, therefore, the principal object of the present invention to find a suitable procedure for removing one carbon atom fromv the isopropyl side chain of dimethyl styrene,

CE: I to convert the latter to p-methyl styrene,

when the latter is the preferredpr'oduct. In genisolating the p-methyl styrene content thereof.

' Dimethyl styrene, which has the .iormulais so nearly analogous in structure of p-cymene,

/CH: H8000 Subj ecting p-cymene to thermal cracking requires a temperature of above about 600 C. to produce any appreciable breakdown of the cymene. Tem-v peratures of about 700 are required to produce effective conversions. At about this temperature,

yields up to about can be obtained although at the expense of a material loss of about onethird Ofthe feed. Application of thermal cracking to dimethyl styrene indicates that it is even is more difllcult to decompose since temperatures up to 650-700" C. are required to produce any I appreciable thermal cracking.

Attempts to reduce the thermal decomposition losses in cracking p-cymene by the use of a crack- 20 ing catalyst such as arclay or a silica gel to reduce the temperature is not a suitable solution. The catalytic cracking of cymene produces toluene in high yields as the principal product. Apparently the tendency is for the molecule to break down along the bond :r-r of the formula Catalytic cracking of dimethyl styrene would not be expected to result in a useful procedure since cleavage of the corresponding bond in dimethyl styrene would obviously prevent the formation of any styrene. Surprisingly, however, it has been found that by using a proper catalyst and suitable reaction conditions dimethyl styrene can be catalytically cracked in the vapor phase to produce p-methyl styrene at temperatures low enough 40 to prevent excessive material losses.

Selection of a suitable catalyst for use in the process is not a simple problem. Because of the difllculty encountered with silica catalysts which 'Produced toluene in cracking cymene, itwould be supposed that a different type of catalyst such as a metal or metal oxide would be required. In attempts to carry out the process using catalysts of this type, the first result was to crack the dimethyl styrene to a mixture of products including hydrogen followed by a hydrogenation of dimethyl styrene to cymene. These catalysts, therefore, are not particularly suitable.

Surprisingly, according to the present invention. the same type of siliceous catalysts, compristhat it is to be expected that the two materials 5 ing clay and/or silica which convert p-cymene to g toluene are particularly well suited for the crackin: of dimethyl styrene. Apparently they prorather then along the bond x-u, where, as was noted above, cleavage occurs in the case of P- cymene.

A particularly useful siliceous catalyst was found to be onein which a major proportion comprises silica gel intimately admixed with a minor proportion of from about -30% of a metallic oxide such as that of aluminum. Other oxides which appear to be useful include those of zirconium, beryllium and titanium. The catalyst may'be readily prepared in any one of a number of different ways. Although .the various compolittle H: formation thus making isolation simple.

As compared with the 600-700 required for thermal cracking of cymene and/or dimethyl styrene, the present process is shown to be truly catalytic by the lower operating temperatures renents of the combined catalyst in gel form may be precipitated separately, concurrently or consecutively, the eflectiveness of the catalyst appears to be somewhat altered by the procedure by which the gels are initially associated. Perhaps, because it produces a somewhat more uniform mixture, concurrent precipitation appears to produce a preferable catalyst mass. One precaution should be taken if alkali metal salts are used in the preparation of the gels. Alkali metals or the oxides tend to promote undesirable formation of by-products. Considerable care should therefore be taken to free the catalyst body as far as practically possible from any remaining traces of the alkali metals. Ordinarily this is readily accomplished by washing.

In use, the catalyst may be in somewhat different physical states depending upon the method of precipitating the gels. If produced as solid pieces, the catalyst body may be broken up into suitable pieces, ordinarily from about 6-14 mesh. Preferably, however, it may be broken into finer pieces or precipitated in powdered form. In such cases, the catalyst may be pelleted into suitable sizes for use in the present process. This enables easier packing and more adequate control of the area and time of contact. one-quarter inch pellets were found to be suitable sizes. In developing the present process, it was found that about 515% of alumina gel, the remaining 85-90% being silica gel, produced a particularly eifectlve catalyst.

In carrying out the process some hydrogen is One-eighth to v necessarily formed during cracking. Under the influence of the catalyst a part of the hydrogen appears to be concurrently utilized to convert some of the p-methyl styrene to ethyl toluene. In fact, if the contact time is adequate, then the temperatures used in cracking are high enough to use up nearly all of Hz to form ethyl toluene. This, however, is not a practical commercial limitation since ethyl toluene is itself a commercially useful product, is easily separated from the reaction products, and may be readily converted by well-known dehydrogenation procedures to additional quantities of p-methyl styrene. Actually it is an advantage in that a product of high purity is thereby quite readily obtained.

Under some circumstances cymene would also be produced by a similar hydrogenation of di- While the removal of initial reaction cycle, increasing converted to the vapor state and passed quired. Some cracking is apparent as low as '350-400" C. and useful yields begin to be obtained at about 400-475" C. -Above about 525 C. thermal decomposition of the products begins to be troublesome, resulting in smaller overall recoveries. For optimum operation, the temperature range should bepreferably from about 350-500 for most catalyst compositions in the aluminasilica ratios of 1:20 to about-1:7.

Obviously the'space-velocity over the catalyst has an effect on the nature of the products. Too low a rate results in excessive cracking and rchydrosenation. Too high a rate results in incomplete conversion. The exact numerical limits for all cases cannot be determined since the optimum depends upon the amount of catalyst, theparticle size of the catalyst, the feed rates necessary to obtain production, the temperature, the catalyst composition and the like. However, the opt.mum for any set of conditions maybe readily determined by fixing the other conditions and varying the feed rate.

Preferably, but not necessarily, the vaporized dimethyl styrene should be preheated. This was found to have a stabilizing effect on the whole both the total throughput for any one cycle and reducing the material losses through decomposition. The latter appeared to have a relationship to the temperature change .which the vapors undergo while in contact with the catalyst. Preheating may be done either in a separate chamber, or in an initial portion of the catalytic converter. In the latter case, the preheating zone may be packed with catalytically inactive material if so desired.

The process of the present invention is not necessarily limited to the use of any particular apparatus. It is necessary that the material be over the catalyst at a rate depending upon the temperature and the volume of the catalytic mass. The reacted vapors may be collected by condensa-' tion and separated into their component parts as by fractional distillation. So long as these functions are carried out the exact structure of the apparatus may be varied at will without departing from the scope of the present invention. Nor is the .process limited to any particular materials from which the apparatus is to be constructed. Much of the development work was carried out using stainless steel reaction vessels.

However, any material which is catalytically inactive, does not contaminate the materials and is resistant to intergranular attack by hydrogen liberated during the reaction may be used.

Where the apparatus is available for use, distinct improvements can be obtained by several modifications. Perhaps the simplest of these, although far from the least important, is the provision of an apparatus for condensing the reacted vapors as quickly as possible. Both dimethyl styrene and p-methyl styrene are capable of polymerizingirapidly at these temperatures.

both with themselves-and as copolymers. Therefore, in general the more rapid the condensation 1a the more emcient the production.

I Further improvements. both 'in the ease of operation and in the quality of the product, may be obtained by using ,the catalyst under suitable "fluid flow conditions!" Several different types 01' apparatus are commercially available for this purpose. In general they all provide some means of passing the catalyst mass in a moving stream I In that way the most active fresh catalyst isavailable counter-currently to the flow of vapors.

at the point where it is most needed i. e., approximately'the point at which the vapors leave the reaction chamber to pass into the condenser.

The present invention will be more fully illustrated in connection with the following examples which. are illustrative-only. and not by way of limitation. All parts are by weight unless otherwise noted.

Example 1 analysis was Per cent Dimethyl styrene 45 3 p-Methyl styr 17 Ethyl toluene 15 Cymene 7 Example 2 880 ml. of the pelleted catalyst of Example 1 was set up in a conversion chamber and dimethyl styrene of Np =1.533 was vaporized at the rate of 12 cc./min., pre-heated and passed over the catalyst at about 500 C. Over a three hour run, about 1.5 cu.1t./hr. of gas was evolved and condensate amounting to 67% of the feed was recovered. The product averaged about 32% p-methyl styrene, 35% dimethyl styrene, 6-8% cymene and about 20-22% ethyl toluene.

Ezample 3 'The catalytic mass of Example 2 was reactivated by being burned with air and additional dimethyl styrene of Nn==1.533 was vaporized at a rate of 20 ml./min., pre-heated and passed over a catalyst at 550 C. The oil gas amounted to about 0.5 cu.ft./hr. and the condensate, amounting to about 85% of feed, had a Np"=l.527-1.529, containing about 28% methyl styrene, 47% dimethyl styrene, 4% cymene and 10% ethyl 'toluene.

Example 4 By way of comparison, withthermal' cracking, dimethyl styrene having an Np otabout 1.533 was vaporized at rates varying from 10-25 ml./min. and passed through a thermal cracking zone maintained at about 600 0. methyl styrene was formed,- the maximum amounting to not more than 6% for the best run.

I claim:

1. In a method of converting 4-methyl, alphamethyl styrene to p-metlrvl styrene in the vapor phase, the steps of vaporizing the ii-methyl,

alpha-methyl styrene, preheating the vaporized material to about 300-500 C. and passing the preheated vapors over a siliceous catalyst maintained at from about 400-575 C.

2. A method according to claim 1 in which the siliceous catalyst comprises an intimate mixture of anhydrous alumina and silica gels, the alumina-silica ratio'beinginthe range of from about 1:20 to about 1:7 and containing substantially no alkali metal residues.

3. In a method of converting 4-methyl, alphamethyl styrene to p-methyl styrene in the vapor phase, the steps of vaporizing the Q-methyl, alpha-methyl styrene, preheating the vaporized material to about 300-500 C., and passing the preheated vapors over a siliceous catalyst maintained at from about 400-575? C., and condensing the resultant vapors prior to the polymerization of the styrene derivatives contained therein and isolating the substantially unpolymerized 4- methyl, alpha-methyl styrene content in the condensate. V

JAMES KENNETH DIXON.

Very little 

